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The impact of hypocapnia on outcome in aneurysmal subarachnoid hemorrhage (SAH) is unclear, although hypocapnia is associated with poor outcome in other brain injuries. We sought to determine the incidence and impact of hypocapnia in mechanically ventilated patients with aneurysmal SAH.We assembled a retrospective cohort of 102 consecutive mechanically ventilated patients with aneurysmal SAH admitted to an academic neurosurgical intensive care unit (ICU). Ventilation records, arterial blood gas data, and clinical outcomes were reviewed. The primary outcome was 3-month Glasgow Outcome Scale, with secondary outcomes of ICU and hospital mortality and symptomatic vasospasm.Hypocapnia was common (92% of patients had 1 or more PaCO2 measurements <35 mm Hg), with 68% of these measurements occurring while breathing spontaneously with minimal ventilator support. Median duration of hypocapnia was 4 days (interquartile range, 2 to 12). Forty-eight percent of all PaCO2 measurements on a given day were below 30 mm Hg. Unfavorable outcome (Glasgow Outcome Scale <4) occurred in 52 of 89 patients (58.4%). ICU and hospital mortality was 26.5% and 32.4%, respectively, and 34% developed symptomatic vasospasm. Duration of hypocapnia was associated with unfavorable outcome (adjusted odds ratio 1.33 for each additional day of hypocapnia) and symptomatic vasospasm (adjusted odds ratio 1.25 for each additional day of hypocapnia), but not ICU or hospital mortality. These associations appeared robust in sensitivity analyses to address potential misclassification and ascertainment bias.Hypocapnia is common in ventilated patients with aneurysmal SAH, and a significant proportion of this developed spontaneously despite minimal ventilator support. The duration of hypocapnia is independently associated with poor functional outcomes and symptomatic vasospasm. Further study is warranted to confirm a causal link between hypocapnia and poor outcomes, and to confirm whether tight control of PaCO2 might improve outcomes in aneurysmal SAH.